Electrically conductive polymer based compositions are used in many industrial applications, such as for dissipating electrostatic charge from plastic parts and plastic boxes for shielding electronic components from electromagnetic interference (EMI). Examples for electrostatic discharge (ESD) applications are electronic packaging, clean room installations, storage trays, water carriers, chip carriers and construction components for explosion-proof environments.
Compounds tailored for dissipating static electricity having a typical surface resistivity of 10.sup.2 to 10.sup.13 ohm/square and compounds specified for EMI shielding applications typically exhibit volume resistivity of 10.sup.-2 to 10.sup.2 ohm-cm.
There are known in the art polymer based compounds having appropriate resistivity for both static electricity dissipation and EMI shielding. One class of compounds known in the art is based on polypropylene (PP) or polyethylene (PE) with high carbon loading levels up to 40 to 60% by weight characterized by surface resistivity of about 10.sup.3 -10.sup.8 ohm/square. For applications requiring EMI shielding, weight loading levels of 30 to 50% PAN carbon fibers, 40% aluminum flakes, 15% nickel-coated carbon fibers or 5 to 10% stainless steel fibers have been used for the same class of polymers.
The method currently used to increase the electrical conductivity of polymers is to fill them with specific conductive additives, such as metallic powders, metallic fibers, carbon black, carbon fibers and recently by intrinsically conductive polymeric powders. The characteristic behavior of these materials is the existence of a strongly non-linear relationship between the electrical conductivity and the filler concentration. At low filler loading, the electrical conductivity of the polymeric compound is generally quite low; its magnitude is similar to that of the polymer matrix (10.sup.-16 to 10.sup.-11 ohm.sup.-1 cm.sup.-1). As loading is increased, the conductivity increases sharply by several orders of magnitude over a narrow concentration range, then slowly increases towards the conductivity of the condensed filler powder of the order of 10.sup.-4 to 10.sup.-1 ohm.sup.-1 cm.sup.-1. This behavior describes an insulator-conductor transition occurring at a critical volume fraction (percolation threshold). This threshold is due to the formation of a chain-like network of particles extending throughout the entire specimen volume and allowing electrical current to flow.
U.S. Pat. No. 4,169,816 describes an electrically conductive single thermoplastic material composition with a high carbon content, the composition including for each 100 parts of polypropylene-ethylene copolymer 15-30 parts of carbon black, 0.25 to 1 part of silica and 1-10 parts of a fiber reinforcing agent selected from carbon fibers or a mixture of carbon fibers and glass fibers.
U.S. Pat. No. 5,004,561 describes another single thermoplastic based electrically conductive composition with a high carbon content, the composition including for each 100 parts of thermoplastic resin selected from the group of polyolefin, polystyrene and acrylonitrite/styrene/butadiene (ABS) copolymer resin, polybutylene terephthalate (PBT) resin, polyphenylene ether and polyamide (PA) resin, 30-300 parts of electrically conductive glass fibers, 5-40 parts of carbon black and 5-40 parts of graphite.
Russian Patent No. SU 1,643,568 describes a high carbon based electrical conductive thermoplastic composition in which electrical conductivity is achieved from the dispersion of carbon in the matrix. The composition includes 20-35 weight percent polypropylene, 10-20 weight percent polyamide, 20-30 weight percent carbon black, 10-20 weight percent graphite and 15-20 percent glass fibers.
There are generally two methods for producing electrically conductive thermoplastic articles known in the art. In the slow production rate compression molding method less filler (e.g. carbon black) is required to achieve a desired conductivity, however the mechanical properties of the composition are usually deficient. In the fast production rate injection molding method better mechanical properties are achieved and articles having complex geometry can be produced but the amount of conductive filler required is high. One deficiency of compression molding of electrically conductive compounds is that the relatively slow processing is expensive.
A major disadvantage of prior art polymer based compounds for electrostatic dissipation and EMI shielding applications is the high percentage of conductive additives required to form the conductive polymer compounds resulting in high cost and deficient processability and mechanical properties and also high carbon contamination which is adverse in particular for clean room applications.